Change the parent class of SMCUpdater

[ihincks]

A bunch of the functionality of SMCUpdater is not directly related to the SMC algorithm itself. For example, some methods (ex. sample, est_mean()) could be included in a new ParticleDistribution distribution (remember that SMCUpdater inherits from Distribution).

I bring this up because I am considering different updaters which wouldn't naturally inherit from SMCUpdater, but would benefit from large chunks of the code.